Working method
Delete the existing windows.phone.toolkit in references..
Right click on references.. Select Manage NuGet Packages
Go online and search for windows phone toolkit.. and install it,
then add
xmlns:toolKit="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone.Controls.Toolkit"
in your xaml file on the top.
then type
<toolKit:you will get it
Dont Follow this method
** Don't search for your control in toolbox and drop in UI .. it will cause same error , it will generate a .dll under references